In the following description the term “turbine” is used to refer to rotary engines having a rotating part and a stator part force coupled by a fluid medium such as water, steam or gas. Of particular interest for the present invention are axial turbines comprising radially arranged fixed stator blades or vanes alternating with radially arrangements of moving rotor blades. Movements are generally defined as movements relative to a casing or housing.
In large turbines, particularly steam turbines, the moving blades or airfoils are presently manufactured using steel or titanium based alloys. In a multi-stage turbine, the size of the blades increases from stage to stage. In the final stage of the largest low pressure turbines the height of a turbine blade can exceed one meter or more. While it is desirable to increase the size of the turbine stages and thereby increase its flow-off surface and efficiency, the properties of current materials have reached theirs limits mainly because of the large centrifugal forces acting on the rotating blades.
To overcome the barriers set by the materials properties of steel and titanium, composite material airfoils have been proposed using mainly carbon fiber based materials. Though a large number of such designs have been published, real-world applications of such composite blades are currently limited to gas turbines for advanced aircrafts engines.
One of the reasons which so far prevented large-scale adoption of composite blades in the field of electrical power generation is the lack of resistance of the composite materials to erosion. Specifically in the field of steam turbine blades, the material is subject to erosion by water droplet condensing from the steam passing through the turbine over a long period of operation. Under the constant bombardment of the condensate from the water steam, composite material erodes much faster than the currently applied metal alloys and is thus not suitable as airfoil material for large steam turbine blades.
As known solution to the general problem of erosion, the use of protective layers has been suggested since the early thirties in a number of published patent documents such as the German patent DE 536278 C2. For composite blades, protective layers are described for example in published United States patent application US 2008/0152506 A1 and published international patent applications WO 2011/039075 A1 and WO 2010/066648.
Whilst the solution of applying a protective layer or coating may have the potential of reducing erosion at the exposed parts of the turbine blade, further improvements are required to render composite airfoils operational. In particular, it is seen as an object of the present invention to improve the way the protective layer and the composite core of a turbine airfoil are joined.